A United Airlines Holdings Inc. policy requiring flight attendants to provide a doctor’s note when calling in sick on certain days is being reviewed by the US Labor Department to see if it conforms to federal rules.

The department said it had received “numerous recent inquiries” from workers about the change, the agency said Thursday in a statement. United told employees that starting July 21 it would require the extra documentation until further notice for those calling out sick for assigned flights or being on reserve for Friday, Saturday or Sunday. 

“We have seen a significant increase in sick calls over weekends this summer,” United said in an employee message seen by Bloomberg. Those requests have risen as much as 23%, suggesting “that some flight attendants are misusing sick leave.” The policy also requires those calling out to visit a doctor in person.

The change doesn’t prevent taking approved unpaid leave, and complies fully with federal law and the carrier’s union contract, United said in an emailed statement.

“The policy is narrowly tailored to the days when abuse has been occurring, and we hope to return to our usual approach of not requiring a doctor’s note soon,” the airline said.

The Association of Flight Attendants-CWA, which represents United crew members, has filed a grievance against the airline over the issue, saying it violates their contract is “burdensome and costly” and intended to discourage workers from taking medical leave. The union and airline are currently in negotiations for a new labor agreement. 

In the message announcing the policy, United cited a greater risk of disrupting operations when more employees than normal are off the job for illness. The rule also applies to those taking paid or unpaid sick time, or using sick leave to care for a spouse or child. Failure to submit the note from an accredited physician within 72 hours could result in disciplinary action or termination, the airline said.

The Labor Department’s Wage and Hour Division enforces the Family and Medical Leave Act, which restricts companies from punishing or preventing workers from taking unpaid leave. The agency can pursue damages for violations, and the government or affected employees can bring cases in court.